The present invention relates to a silicon single crystal used for a semiconductor integrated circuit device and to a silicon wafer and an epitaxial wafer, which are obtained therefrom and used for forming an integrated circuit. More particularly, the present invention relates to a silicon single crystal, a silicon wafer, and an epitaxial wafer exhibiting high gettering capability which is provided by doping with nitrogen solely, or with nitrogen and carbon and/or boron during growth of a single crystal and without provision of an additional step.
As the integration density of silicon semiconductor integrated circuit devices rapidly increases, a silicon wafer from which devices are formed is subjected to increasingly severe specifications. Thus, since circuits are made thinner with increasing integration density within a device active region wherein a device is formed on a wafer, crystal defects, such as dislocations and elemental metal impurities other than a dopant, which increase leakage current and shorten the life of a carrier are subjected to more rigorous limitations than ever before.
Conventionally, a wafer produced by slicing a silicon single crystal obtained through the Czochralski method has been used for a semiconductor device. Generally, the wafer contains oxygen at a concentration of about 1018 atoms/cm3. Although oxygen is effective for enhancing the strength of a silicon wafer by preventing generation of dislocations and for providing a gettering effect, oxygen is well known to deposit in the form of an oxide and to induce crystal defects such as dislocation or a stacking fault caused by heating during production of a device. However, in a process of device production, a defect-free DZ layer (denuded zone) having a thickness of about 10 xcexcm is formed near the wafer surface by diffusion of oxygen to the outside, since the wafer is maintained at a temperature as high as 1100-1200xc2x0 C. for several hours so as to form a field oxide film through LOCOS (Local Oxidation of Silicon) and a well diffusion layer. The denuded zone serves as a device active region, to thereby provide a reduction in crystal defects.
However, in conjunction with the increasing density of integration, a high-energy ion implantation method has been employed for forming a well, and a device has been produced at a temperature of 1000xc2x0 C. or less. Therefore, oxygen diffuses slowly, and formation of the above-mentioned denuded zone is insufficient. Even though reduction of oxygen content in a substrate has been attempted, crystal defects are insufficiently suppressed and the performance of a wafer is deteriorated by the reduction in oxygen content. Thus, attempts to reduce oxygen content have yielded unsatisfactory results. Therefore, an epitaxial wafer wherein a silicon epitaxial layer containing substantially no crystal defects has been formed on a silicon slice serving as a wafer substrate has been developed and is widely used for a large-scale integrated device.
Thus, feasibility of complete prevention of crystal defects in a device active region on a wafer can be enhanced by employment of an epitaxial wafer. However, contamination with elemental metal impurities exerts a strong influence, because a complicated process is required for realizing high-density integration and contamination occurs frequently. Although purification of the production environment and raw materials is essential for preventing contamination, complete prevention of contamination in the process of producing the device is difficult. Therefore, gettering is employed. Gettering is a method in which impurity elements provided through contamination are collected outside the device active region so as to eliminate negative influences.
Elemental metal impurities diffuse into a silicon crystal at a relatively low temperature, to thereby form a solid solution, and generally diffuse in silicon at high speed. When crystal defects such as dislocation and distortion caused by fine deposits occur, the impurities tend to concentrate to the defects, in order to attain a more stable energy state than that in the case where impurities exist in the crystal lattice. Therefore, a crystal defect is intentionally introduced to thereby capture and confine impurities. The site where the impurities are captured is called a sink. Sinks are produced by two types of gettering methods; i.e., extrinsic gettering and intrinsic gettering.
Extrinsic gettering is a method in which crystal defects are introduced by means of distortion induced by extrinsic factors such as sandblasting, polishing, laser radiation, ion implantation, and growth of Si3N4 film or polycrystalline Si film; whereas intrinsic gettering is a method in which a number of micro-defects, which are probably induced by oxygen while a wafer obtained through the Czochralski process involving oxygen is alternately subjected to high-temperature heat treatment and low-temperature heat treatment, are employed as sinks.
Of the above-mentioned gettering techniques, extrinsic gettering represented by imparting distortion to a reverse side of a wafer involves drawbacks such as an increase in production costs due to addition of production steps; generation of particles due to detachment of silicon chips from a portion imparted with distortion; and warp of a wafer resulting from the treatment.
In intrinsic gettering, heat treatment is required for effective production of sinks, and therefore intrinsic gettering requires additional steps. Furthermore, in an epitaxial wafer substrate, oxide precipitates which are to serve as nuclei of micro-defects shrink to disappear due to employment of a temperature as high as 1050-1200xc2x0 C. during a step for forming an epitaxial layer, to thereby disturb subsequent formation of sinks during heat treatment. Particularly, as mentioned above, when a device process is carried out at relatively low temperature, the growth rate of oxide precipitates decreases to disadvantageously result in an insufficient gettering effect to metal impurities at an initial stage of the device process as well as during the entire course of the step.
To overcome these drawbacks, there has been a method employed in which a wafer is thermally treated before and after an epitaxial process in order to intentionally generate crystal defects which getter impurities. Conventionally, a number of gettering methods have been proposed. However, other drawbacks remain, such as a long-duration heat treatment and complex processing steps.
For example, Japanese Patent Application Laid-Open (kokai) No. 3-50186 discloses a method in which a heat treatment is carried out at 750-900xc2x0 C. before an epitaxial process to thereby ensure generation of oxide precipitates. Although the specific temperature for the heat treatment is not specified, based on assumptions that follow from the description, the heat treatment might be required for as long as four hours or more. Japanese Patent Application Laid-Open (kokai) No. 8-250506 discloses a method in which one-step or two-step annealing at low temperature is carried out; the annealed wafer is maintained within a medium temperature range; and subsequently epitaxial growth is carried out. Furthermore, Japanese Patent Application Laid-Open (kokai) No. 10-229093 discloses a method comprising treating a wafer sliced from a crystal doped with carbon at a concentration of 0.3xc3x971016 to 2.5xc3x971016 atoms/cm3 at 600-900xc2x0 C. for 15 minutes to four hours; polishing one or both surfaces of the wafer; and carrying out epitaxial growth.
With regard to a heat treatment after an epitaxial process, Japanese Patent Application Laid-Open (kokai) No. 63-198334 discloses a method in which annealing is carried out at 650-900xc2x0 C. for as long as 4-20 hours, or stepwise temperature elevation between 650xc2x0 C. and 900xc2x0 C. is carried out after an epitaxial process to thereby ensure generation of oxide precipitates. Japanese Patent Application Laid-Open (kokai) No. 63-227026 discloses a method in which carbon is doped at a high concentration while a crystal is being pulled; epitaxial growth is carried out; and two-step heat treatment i.e., low temperature annealing and medium temperature annealing, is carried out to thereby ensure generation of oxide precipitates. The method also requires a heat treatment of eight hours or longer.
As described hereinabove, a heat treatment carried out before and after an epitaxial process may introduce problems, such as decrease in productivity and increase in costs due to an increase in the number of steps; damage to a boat during the treatment; and a reduction in yield due to particle generation. Moreover, since a variety of device processes are carried out after an epitaxial process and the history of the heat treatment of a wafer varies in accordance with the device processes, formation of oxide precipitates, growth of the precipitates, and gettering capability induced thereby also vary. Therefore, heat treatment conditions must be selected in accordance with the device processes.
To overcome the above drawbacks involved in production of a silicon single crystal, a silicon wafer, and an epitaxial wafer, an object of the present invention is to provide a silicon single crystal characterized in that precipitates which are not extinguished even during a high-temperature epitaxial process are formed therein without performance of extrinsic or intrinsic gettering treatment, which is a factor for increasing costs, and in that a gettering effect thereof is stable during any subsequent device process involving any temperature profile. Another object of the present invention is to provide a silicon wafer obtained from the silicon single crystal. Still another object of the present invention is to provide an epitaxial wafer produced from the silicon wafer.
Oxidation-induced stacking fault (hereinafter referred to as simply xe2x80x9cOSFxe2x80x9d) is one type of fine crystal defect attributed to contained oxygen. OSF is a stacking fault generated in a crystal under an oxide film during a high-temperature oxidation treatment in a device process. Generation of OSF exhibits positive correlation with the content of oxygen in a Si crystal. The defect is grown from oxide precipitates serving as growth nuclei. When a Si single crystal wafer produced through the Czochralski method is treated at 1000-1200xc2x0 C. for 1-20 hours, ring-like distributed oxidation-induced stacking faults (hereinafter referred to as xe2x80x9cOSF ringsxe2x80x9d) may be generated around the axis along which the single crystal is pulled. The present inventors have found that a Si epitaxial layer is formed on a substrate including OSF rings and that oxide deposits within a ring region function as effective gettering sites without being extinguished during a production step of a device performed after epitaxial growth.
In general, an OSF ring has a width of some mm to some tens of mm and a boundary between an OSF ring and an adjacent region is distinctly defined. When a crystal is pulled at a high pulling speed, the diameter of the ring increases to approximately the outer diameter of a wafer, whereas when the pulling speed is reduced, the OSF rings are gradually reduced in diameter and eventually extinguished.
In consideration of the gettering effect induced by crystal defects in an OSF ring region, the present inventors have conducted a variety of studies directed toward conditions that increase the width of an OSF ring, and have found that doping of nitrogen during Czochralski growth of a single crystal increases the width of the ring. Thus, when the entire surface of a wafer serves as an OSF region, nuclei of precipitates that are difficult to extinguish during an epitaxial process and stable at high temperature effectively function as gettering sites.
Effects of nitrogen doping during Czochralski growth of a single crystal have conventionally been known. For example, Japanese Patent Application Laid-Open (kokai) No. 61-17495 discloses an effect for strengthening a crystal; Japanese Patent Application Laid-Open (kokai) No. 60-251190 discloses an effect for preventing generation and movement of dislocation induced by thermal stress; and Japanese Patent Application Laid-Open (kokai) No. 5-294780 discloses an effect for preventing generation of etch pits in a wafer and a decrease in gate oxide integrity of a device. However, such disclosed methods are directed toward preventing dislocations or preventing deterioration in withstand voltage, and effects of these methods on gettering and the shape of OSF rings have remained unknown.
Thus, the present inventors have studied conditions for increasing the width of OSF rings and generating crystal defects attributed to the rings on the entire surface of a wafer, as well as for increasing the effectiveness of the gettering effect, and have found that when nitrogen serves as a single dopant and is doped in an amount of 1xc3x971018 atoms/cm3 or more, nuclei of OSF are produced and diffused in an amount effective for attaining homogeneous gettering in a single crystal. In addition, when a Si epitaxial layer is formed on the surface of a slice obtained from the single crystal, there is produced a wafer having very few surface defects and exhibiting effective gettering action in a step for producing a device.
The concentration of nitrogen doped into a wafer is calculated from the amount of nitrogen doped in silicon before pulling; the distribution of nitrogen in a silicon melt and in solid; and the degree of solidification of the crystal. Briefly, the initial concentration of nitrogen in silicon, C0, is calculated from the amount of silicon atoms in a raw material and the amount of nitrogen atoms added, and the concentration of nitrogen in the crystal CN is calculated by use of the following equation (a):
CN=C0k(1xe2x88x92x)kxe2x88x921xe2x80x83xe2x80x83(a)
wherein k is the equilibrium segregation coefficient of nitrogen, which is 7xc3x9710xe2x88x924, and x is the degree of solidification, which is represented by the weight of the pulled portion of a crystal divided by an initial charge weight.
The above-described gettering method is particularly effective for wafers used in a pxe2x88x92, nxe2x88x92, or n+ device in which precipitate nuclei for forming sinks are easily extinguished by a step for forming an epitaxial layer. In addition, the method is also effective for a p+ wafer doped at high concentration with boron which getters Fe and effectively getters an element other than Fe.
The gettering effect for the epitaxial-layer-formed wafer is evaluated by MOS generation lifetime. The present inventors have conducted further, detailed investigation of wafers exhibiting excellent results among the thus-nitrogen-doped wafers, and have found that generation of OSF is observed at a density of 102/cm2 or more at a surface of substrate after a thermal oxidation treatment. Briefly, when a single crystal possesses defect nuclei, which produce OSF at a certain density or more through the thermal oxidation treatment, an excellent gettering effect may be attained.
The epitaxial layer is preferably formed on a wafer, which is heated to 1000xc2x0 C. or higher. When a wafer sliced from a nitrogen-doped single crystal is heated to 1000xc2x0 C. or higher, a temperature similar to that used for formation of the epitaxial layer, defects are observed at a density of 5xc3x97104/cm2 or more in a cross-section. Such defects serve as sinks for gettering to thereby enhance the gettering effect of a wafer, and are obtained from defects nuclei generated in a single crystal by nitrogen doping.
However, a variety of device processes are carried out after an epitaxial process, and the history of the heat treatment of a wafer varies in accordance with the device processes, such as a low-temperature device process which is mainly carried out at a temperature of 800xc2x0 C. or less, and a high-temperature device process which is mainly carried out at a temperature greater than 800xc2x0 C. When a low-temperature device process is employed, oxide precipitate nuclei, which are not extinguished during an epitaxial process but remain thereafter, grow at a speed lower than that in the case of a high-temperature device process, to thereby yield insufficient gettering capability. In order to solve the problem, the present inventors have found that carbon or boron, which enhance the formation rate and the growth rate of oxide precipitates, is doped in addition to nitrogen even in a low-temperature device process, to thereby ensure excellent gettering capability.
The present invention has been accomplished based on this finding, and comprises three aspects, i.e., (1) a silicon single crystal, (2) a silicon wafer, and (3) an epitaxial wafer.
Accordingly, in aspect (1) of the present invention, there is provided a silicon single crystal suitable for production of an epitaxial wafer characterized in that the single crystal is grown with nitrogen doping at a concentration of 1xc3x971013 atoms/cm3 or more, or with nitrogen doping at a concentration of 1xc3x971012 atoms/cm3 and carbon doping at a concentration of 0.1xc3x971016xe2x88x925xc3x971016 atoms/cm3 and/or boron doping at a concentration of 1xc3x971017 atoms/cm3 or more.
In aspect (2) of the present invention, there is provided a silicon wafer that is produced by slicing the silicon single crystal described in aspect (1).
In aspect (3) of the present invention, there is provided an epitaxial wafer in which an epitaxial layer is grown on a surface of the silicon wafer described in aspect (2).
Preferably, the epitaxial wafer has an oxygen concentration of 12xc3x971017 atoms/cm3 or more when the wafer is subjected to a device process carried out at 1100xc2x0 C. or higher after epitaxial growth.
Preferably, the epitaxial wafer is characterized in that an epitaxial layer is grown on a surface of a single crystal wafer which is sliced from a silicon single crystal grown accompanied by nitrogen doping and generates OSF at a density of 1xc3x97102/cm2 or more through a thermal oxidation treatment.
Preferably, the epitaxial wafer is characterized in that a single crystal wafer is sliced from a silicon single crystal grown accompanied by nitrogen doping and generates defects at a cross-sectional density of 5xc3x97104/cm2 or more before epitaxial growth and the epitaxial wafer generates defects at a cross-sectional density of 1xc3x97104/cm2 or more through a thermal treatment carried out at 1000xc2x0 C. or higher.
Preferably, an epitaxial layer is grown on a silicon wafer which is sliced from a silicon single crystal grown accompanied by nitrogen doping at a concentration of 1xc3x971012 atoms/cm3 or more when the epitaxial layer is subjected to a high-temperature device process carried out at a temperature substantially higher than 800xc2x0 C. after epitaxial growth.